It is practically universal practice to monitor the pulse and respiration of a patient anesthetized for surgery. An EKG (electrocardiogram) is continuously displayed on a monitor. Thus, the anesthesiologist and/or surgeon can at all times ascertain the status of the patient's cardiovascular activity. A grounding electrode or "pad" is provided beneath the patient, commonly beneath the buttocks in the supine patient. Additional "pickup" electrodes are applied to the patient's body, commonly on the chest. All of the electrodes are connected to a commercially-available EKG monitor for displaying the pattern of electrical impulses detected as a result of cardiac activity on a cathode ray screen or other visual display device.
In certain types of surgery it is common to apply electrocautery in which a surgical knife is supplied with a relatively high level of radio frequency energy so that blood vessels or other tissue are cauterized and sealed immediately upon cutting. However the presence of such radio frequency signal or energy on the patient's body and particularly in the region of the EKG "pickup" electrodes causes severe interference with the operation of the EKG monitor and resultant electrocardiogram. In fact such radio frequency interference has been known to render the monitor totally inoperable for many seconds, resulting in a loss of the desired continuous observation of cardiovascular activity as reflected by the EKG display.
If electrocautery is practiced with no precautions, the radio frequency signal will force the EKG readings completely "off scale", and will overload the EKG circuits to the point where it may be many seconds before the monitor can recover after the radio frequency energy is turned off. Some effort has been made to correct this situation using an electrical surgical interference supression (ESIS) filter between the electrodes and the monitor. Even with such a filter something on the order of 30% of the radio frequency gets through to the monitor screen. This produces a distorted image on the best of monitors, and sends others completely off scale. Recovery time is somewhat improved due to the diminished radio frequency signal, but still requires several seconds in most cases.